# Who invented radiometric dating definition

1 - Department of Geosciences,

2 - Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

3 - Research Laboratory for Archaeology and History of Art, University of Oxford, Oxford, OX1 3QJ, UK

4 - Institut für Mittelenergiephysik, ETH-Hönggerberg, CH-8093 Zürich, Switzerland

5 - Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964, USA

6 - Research Laboratory, British Museum, London WC1B 3DG, UK

Very small samples from the Shroud of Turin have been dated by accelerator
mass spectrometry in laboratories at Arizona, Oxford and Zurich.
As Controls, three samples whose ages had been determined independently
were also dated. The results provide conclusive evidence that the
linen of the Shroud of Turin is mediaeval.

The most widely known form of radiometric dating is carbon-14 dating . This is what archaeologists use to determine the age of human-made artifacts. But carbon-14 dating won't work on dinosaur bones. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old. Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old. To determine the ages of these specimens, scientists need an isotope with a very long half-life. Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40 , each of which has a half-life of more than a million years.

Unfortunately, these elements don't exist in dinosaur fossils themselves. Each of them typically exists in igneous rock, or rock made from cooled magma. Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock. But this sediment doesn't typically include the necessary isotopes in measurable amounts. Fossils can't form in the igneous rock that usually does contain the isotopes. The extreme temperatures of the magma would just destroy the bones.

So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed. By using radiometric dating to determine the age of igneous brackets , researchers can accurately determine the age of the sedimentary layers between them.

Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world. This information has also helped determine the age of the Earth itself. While the oldest known rocks on Earth are about 3.5 billion years old, researchers have found zircon crystals that are 4.3 billion years old [source: USGS ]. Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.5 billion years old. In addition, the oldest known moon rocks are 4.5 billion years old. Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age.

Radiometric dating isn't the only method of determining the age of rocks. Other techniques include analyzing amino acids and measuring changes in an object's magnetic field. Scientists have also made improvements to the standard radiometric measurements. For example, by using a laser, researchers can measure parent and daughter atoms in extremely small amounts of matter, making it possible to determine the age of very small samples [source: New Scientist ].

Before 1955, ages for the Earth based on uranium/thorium/lead ratios were generally about a billion years younger than the currently popular 4.5 billion years. The radiometric evidence for a 4.5 b.y. old Earth is reviewed and deficiencies of the uranium/lead method are discussed. The basic theory of radiometric dating is briefly reviewed. Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth. In 1972 this assumption was shown to be highly questionable.

Despite this, the momentum gained in the two decades prior to 1972 has made 4.5 b.y. a popularly accepted “universal constant” even though the foundations on which it was based have been virtually removed. Some evidence is also presented to show that radiometric results that are in agreement with the accepted geological time scale are selectively published in preference to those results that are not in agreement.

The geological time scale and an age for the Earth of 4.5 b.y. rely heavily on the uranium/thorium/lead radiometric dating methods. 1 , 2 , 3 Because it is not generally appreciated that the assumptions on which the radiometric estimates are based are a virtually impossible sequence of events, let us refresh our minds on the fundamentals of the method by turning to the hourglass analogy (Fig. 1). This system of measuring time works well providing that:

Since radioactive decay constants are believed to be unalterable, the requirement of an absolutely reproducible rate is hopefully met. Therefore, all one has to do in general terms is to find a radioactive mineral which has been a closed system since the time of mineralization, and for which the amount of the daughter product at the beginning is known, the so-called primordial amount, and the absolute age may be calculated from the present amount of parent and daughter isotopes in the mineral.

Briefly, the weakest points in this method are that (a) truly closed systems probably do not exist in nature, 4 (b) the primordial concentration of isotopes is an intractable problem and the value chosen can only be based on assumptions and (c), even the invariance of decay constants is now under question. 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12

More than a dozen radioactive isotopes are known to have easily altered decay constants, by up to 4% 13 by merely changing the chemical form of the isotope. Therefore, the following is simply a statement of the obvious;

Before we consider the actual lead/lead isotope data there is one other comment that needs to be made regarding extrapolation of present rates. The radiometric dating method is basically an extrapolation of the form shown in Fig. 2.

# Crookes radiometer - Wikipedia

The most widely known form of radiometric dating is carbon-14 dating . This is what archaeologists use to determine the age of human-made artifacts. But carbon-14 dating won't work on dinosaur bones. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old. Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old. To determine the ages of these specimens, scientists need an isotope with a very long half-life. Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40 , each of which has a half-life of more than a million years.

Unfortunately, these elements don't exist in dinosaur fossils themselves. Each of them typically exists in igneous rock, or rock made from cooled magma. Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock. But this sediment doesn't typically include the necessary isotopes in measurable amounts. Fossils can't form in the igneous rock that usually does contain the isotopes. The extreme temperatures of the magma would just destroy the bones.

So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed. By using radiometric dating to determine the age of igneous brackets , researchers can accurately determine the age of the sedimentary layers between them.

Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world. This information has also helped determine the age of the Earth itself. While the oldest known rocks on Earth are about 3.5 billion years old, researchers have found zircon crystals that are 4.3 billion years old [source: USGS ]. Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.5 billion years old. In addition, the oldest known moon rocks are 4.5 billion years old. Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age.

Radiometric dating isn't the only method of determining the age of rocks. Other techniques include analyzing amino acids and measuring changes in an object's magnetic field. Scientists have also made improvements to the standard radiometric measurements. For example, by using a laser, researchers can measure parent and daughter atoms in extremely small amounts of matter, making it possible to determine the age of very small samples [source: New Scientist ].